A primary-secondary variable flow design chiller plant includes a piping loop for chillers that is hydraulically separate from a piping loop for the system or load (coils). The piping loop for the chillers is known as the “primary loop” and the piping loop for the system or load is referred to as the “secondary loop.” The two loops share a small section of piping known as “common pipe,” also known as a “decoupler pipe.” Depending on which loop has the greater flow rate, the flow direction in the decoupler pipe changes. The primary loop pumps are usually constant volume, low-head pumps intended to carry a low constant flow through the chiller's evaporator. The secondary loop pumps deliver the chilled water from the common pipe to the cooling loads and then back to the decoupler pipe.
Usually, it is desirable to have the flow rate in the primary loop equal to or greater than the flow rate in the secondary loop. If the flow rate in the primary loop is greater than the flow rate in the secondary loop, then some of the cold water supplied by the chillers is bypassed through the decoupler pipe to the return side of the chillers and the cold bypass water mixes with the return water from the secondary loop and, as a consequence, the temperature of the return water drops. This water is then pumped back to the chiller.
When the secondary flow exceeds the primary flow, return water from the system secondary loop flows back through the decoupler pipe and gets mixed with the supply water from the chillers. The mixed water then loops back through the primary loop. The water that returns through the secondary loop and returns to the secondary loop causes the temperature of the supply water to the secondary loop to increase, which has some unwanted consequences. For example, if the temperature of the water returning from the secondary system is lower than the design temperature, the chillers cannot be loaded to their maximum capacity. This situation is known as “low delta-T syndrome.” This “low delta-T syndrome” results in greater pump, chiller and cooling tower energy consumption, and reduction in cooling plant capacity.